Application Guide for Solving Harmonics Problems

THE NEED TO ADDRESS HARMONICS

One of the two most common problems for industrial power users is striving to meet IEEE Std. 519TM-2014. The IEEE 519 is a standard developed for utility companies and their customers in order to limit harmonic content and provide all users with better power quality. In the future, some utility companies may impose a penalty for users producing harmonics. Some key areas of the standard are as follows in Tables 1 and 2:

TABLE 1
Voltage Distortion Limits

Bus Voltage at PCC*

Individual Voltage
Distortion (%)

Total Voltage
Distortion THD (%)

69 kV and below

3.0

5.0

69.001 kV through 161 kV

1.5

2.5

161.001 kV and above

1.0

1.5

NOTE: High-voltage systems can have up to 2.0% THD where the cause is an HVDC terminal that will attenuate by the time it is tapped for a user.

The other most common problem resulting from harmonics is power factor capacitor failures, or a need to correct power factor in a harmonic environment. In low-voltage systems (600 V or less), capacitors are typically the lowest impedance at harmonic frequencies. Therefore, they experience very high currents and increased heat, which causes them to fail. So, even if meeting IEEE 519 is not the goal, dealing with harmonics may still be required.

When correcting power factor, there is the possibility of creating a resonant circuit. A resonant circuit occurs at the frequency when the impedance of the system (mostly the power transformer) and the impedance of the power factor capacitor are equal. This is called the resonant frequency.

The following formula can be used to find the resonant frequency in terms of harmonic number.

h= resonant frequency in terms of harmonic number

kVAsc= short-circuit capacity at the capacitor

kVAr= rated kVAr of the unswitched capacitance

If there are any sources of current at that frequency, they will be amplified. This will cause high distortion levels and could blow fuses or trip circuit breakers.